U.S. Pat. No. 3,440,333 describes a coil spring that blocks itself in an open state so it is expanded. When strands are pushed into the coil spring and strike against a stop at the end of the coil spring, the blocked state may be lifted and the coil spring may contract. Due to the triangular shape of the wire of which the coil spring is made, the coil spring may dig into the strands and may provide high extraction forces.
But U.S. Pat. No. 3,440,333 has a variety of drawbacks. The strands placed into the coil spring may not be stable enough to reliably trigger the closing mechanism and may potentially fan out under the pressure. The triangular cross-section of the wire of which the coil spring is made may result in convolution impressions on the strands. The surface of the strands may therefore be damaged, which may result in an increased tendency toward corrosion. Moreover, the coil spring may slide away in the housing and not be opened any longer once it has been closed.